System and method for sealing a tank opening

ABSTRACT

System for closing off an opening in a plastic tank, formed from a sheet of multilayer plastic that includes a barrier layer, the said sheet being welded around the perimeter of the opening and provided with a peripheral region that is slimmed down so as to effectively restrict the permeation of gases and liquids into the layer located between the tank and the barrier layer. Method of closing of a tank by welding such a plate, comprising the welding of the slimmed-down peripheral part by the technique of laser or infrared radiation welding. Use of the method to close off a fuel tank for a motor vehicle.

The present invention relates to a system for closing off an opening ina tank.

Liquid and gas tanks in use in industry or on board vehicles of variouskinds must in general satisfy sealing and permeability standardsrelating to the type of use for which they are designed and theenvironmental requirements that they must meet. At the present time,both in Europe and in the world, the requirements regarding thelimitation of emissions of pollutants into the atmosphere and into theenvironment in general are being considerably tightened. The design ofliquid and gas tanks is consequently undergoing rapid developmenttowards using techniques capable of better guaranteeing sealing andsafety under a variety of operating conditions. Moreover, it is alsoendeavoured to reduce as far as possible any losses arising from thevarious accessories and pipes associated with the tanks. One meanssometimes used has been to incorporate certain accessories and pipeswithin the tanks, thus eliminating their interfaces with the externalatmosphere.

It is known, from Patent Application WO 01/21428 by SOLVAY, to close offan opening in a multilayer fuel tank by means of a multilayer plasticsheet welded to the wall of the tank so as to cover the opening, thelayer of plastic of the sheet welded to the tank having a compositioncompatible with the surface layer of the tank.

In the tank described in that patent application, there remains,however, a fuel permeation channel located between the barrier layer,with which the inside of the multilayer structure of the closure sheetsis generally provided, and the wall of the tank, the channel being madeof a plastic such as HDPE, the impermeability to fuel of which is notoptimal.

The object of the present invention is to provide a system for closingoff a tank which further reduces the losses of liquid and gas comparedwith the known closure systems and makes it possible to achieve losslevels low enough to meet the new environmental standards that areprogrammed to come into force in the near future both in Europe and theUnited States.

For this purpose, the invention relates to a system for closing off anopening in a;plastic tank, formed from a plastic sheet welded to thewall of the tank around the perimeter of the opening, the sheet beingformed from a multilayer structure that includes a barrier layer, theouter layer of which structure, welded to the tank, has a compositioncompatible with that of the plastic forming the outer surface of thetank, in which a peripheral region of the sheet, in the weld region, hasbeen slimmed down so as to adapt its geometry according to the followingrelationship:l/e≧5in which l represents the width of the slimmed-down region and e thethickness of the layer in contact with the wall of the tank, bothexpressed in the same length units.

Preferably, the l/e ratio is at least 8.

The term “tank” is understood to mean a closed chamber, of variousshapes, generally sealed with respect to the outside, which may-beequipped with various accessories internal to the chamber or passingthrough its wall. The tank may be a gas tank, a liquid tank or a tankdesigned to contain both a gas and a liquid. Preferably, it is a liquidtank or a liquid tank containing also a gaseous phase.

Certain tanks have one or more openings, that is to say cut-outs ofcircular or other shape, made through the wall of the tank for variouspurposes, for example so as to be able to introduce one-or moreaccessories into the tank that cannot be incorporated therein during itsmanufacture.

Before being able to use the tank for its normal application of storingand confining liquid and gas, it is therefore necessary to design andfit a closure system capable of sealing the tank as hermetically aspossible after the accessory has been introduced.

According to the invention, the closure system is designed for plastictanks. The term “plastic” is understood to mean any synthetic polymericmaterial, being thermoplastic or thermosetting in the solid state underambient conditions, and also blends of at least two of these materials.The polymers intended comprise both homopolymers and copolymers(especially binary or ternary copolymers). Examples of such copolymersare, non-limitingly, random copolymers, linear block copolymers, otherblock copolymers, and graft copolymers. Thermoplastic polymers,including thermoplastic elastomers, are preferred.

The plastic tanks according to the invention are preferably in the formof tanks of multilayer structure. Tanks comprising at least one layer ofhigh-density polyethylene (HDPE) are particularly preferred.Particularly preferred tanks are those in which the structurefurthermore includes at least one internal layer made of a barriermaterial, that is to say a material, generally of polymeric nature,which has a very high impermeability to certain liquids and gases.

For example, it is possible to use known barrier compositions such asthose used for making fuel tanks impermeable. Examples of such barriercompositions are, non-limitingly, resins based on polyamides orcopolyamides, ethylene/vinyl alcohol random copolymers (EVOH), or eventhermotropic liquid-crystal polymers (LCP) such as copolyesters ofp-hydroxybenzoic acid with either 6-hydroxy-2-naphthoic acid orterephthalic acid and with 4,4′-biphenol (for example, the copolyesterssold under the brand names VECTRA® and XYDAR®). The term “sheet” isunderstood to mean any solid element whose thickness is small comparedwith its area and whose rigidity is close to that of the walls of thetank. According to the invention, the material constituting the sheetcomprises a plastic. Preferably, the sheet forming part of the closuresystem according to the invention is produced exclusively from plastic.The sheet may come in various shapes; usually the shape of the sheetdepends on the various types of tank and on the position of the openingin the surface of these tanks that has to be closed off. The sheet oftenhas a plane or slightly curved surface, in particular in the case ofsmall openings such as, for example, circular openings with a diameternot exceeding 80 mm.

According to the invention, the sheet is welded to the wall of the tankaround the perimeter of the opening. By this it should be understoodthat the sheet is fastened to this wall by bringing the molecules of aportion of the surface of one of the two main faces of the sheetadjoining the ends of the latter and forming a junction surface in theform of a closed ring, of circular or any other shape, and to a greateror lesser extent faithfully matching the outer perimeter of this face ofthe sheet, into contact and partial interpenetration with the moleculesof a similar surface of the tank, located around the border of theopening, over its entire perimeter.

According to the invention, the sheet is formed from a multilayerstructure, the outer layer of which, welded to the tank, has acomposition compatible with that of the plastic forming the outersurface of the tank.

The term “compatibility” is understood here to mean chemical andphysical compatibility of the constituents of the respectivecompositions of the layer of the sheet welded to the tank and of theouter wall of the latter in the region of the opening. Goodcompatibility prevents the phenomenon of segregation of certainconstituents of the respective compositions of the welded parts, andlikewise any chemical reaction and physical phenomenon prejudicial tothe properties of the tank whose opening is thus closed off. In general,good compatibility ensures long-term adhesion of the sheet to the tank.

One particular preferred system according to the invention is that inwhich the tank includes, like the sheet, a barrier layer impermeable toliquids and gases. The barrier layers of the sheet and of the tank maybe identical or different. In general, it is preferred for these barrierlayers to be identical, particularly when they comprise identicalbarrier compositions.

These barrier compositions may be incorporated into one of the layers ofthe sheet or a special additional layer essentially consisting of abarrier composition may be inserted into the structure. Preferably, anadditional layer essentially consisting of a barrier composition isinserted into the structure.

When the tank has a multilayer structure, the multilayer structure ofthe sheet may be different from that of the walls of the tank itself,but may also be identical thereto. Closure systems in which thestructure of the sheet is identical to that of the walls of the tank areoften preferred. Most particularly preferred are systems in which,within identical structures, the barrier compositions themselves areidentical and constitute identical polymeric layers.

As a variant, the multilayer structure of the sheet may also comprisetwo different barrier layers. A preferred example according to thisvariant is that of sheets having two complete structures identical tothat of the walls of the tank, stacked one on top of the other andresulting from the compression of two structures identical to the wallsof the tank.

Another variant consists in producing, near the edge of the sheet, anembossment in which the thicknesses of the layers are reduced, inparticular those of the layers other than the barrier layer. The benefitis in this case to further limit the risk of loss by transmission at thepoint where the sheet is welded to the tank.

The expression “outer layer of the sheet” is understood to mean a layerone of whose faces is adjacent to another layer of the sheet and whoseother face is in contact with the environment external to this sheet.

The system according to the invention is very suitable for closing off asimple opening in a tank. It is also suitable for closing off an openingabove an accessory located in the tank.

According to one particularly preferred embodiment of the closure systemaccording to the invention, the sheet is fastened to at least oneaccessory located on the inside of the tank. The term “accessory” isunderstood to mean in general any member via which liquid or gas passes,or one in contact with liquid or gas, and which fulfils a particularfunction specific to the device of which the tank forms part, includinga function of transporting liquid and/or gas between two other members.

Examples of such accessories comprise, not limitingly, the following:

-   -   a container that contains any chemical or physical composition,        especially a vapour absorption canister;    -   a liquid or gas gauge;    -   a electrical connection terminating in a liquid or gas gauge;    -   a liquid or gas pump;    -   a safety valve for controlled closure of the tank under certain        special conditions;    -   a drainable container for receiving liquid;    -   an electrical connection for supplying the motor of a liquid or        gas pump;    -   a liquid pipe terminating in a supply device for any device,        especially an engine;    -   a liquid/vapour separation device.

Any combination of at least two accessories may be used, optionally inthe presence of several examples of the same accessory.

According to this embodiment, the accessory is fastened to the sheet,that is to say it is linked to the sheet by a mechanical means. Theaccessory is fixed to the sheet by any mechanical means compatible withthe type of tank, the nature of the liquid and the gas that are storedtherein and the normal usage to which the tank is put. The sheet mayprovide a means of gripping and transporting the accessory. For example,it is possible to use a sheet to which the accessory is fixed by amechanical fixing device. Such devices are, for example, screws, bolts,nuts, clip systems, etc. It is also possible to use a sheet and anaccessory that have been fastened together right from their manufacture,for example by moulding. It is also possible to make use of a fasteningmeans with a more permanent character, such as adhesive bonding orwelding. In general, it is preferred for the accessories to be fastenedto the sheet by welding.

The accessory fastened to the sheet is present with respect to thelatter on that face of the sheet which is on the inside of the tank whenthe closure has been made.

Accessories of various types fall within this embodiment. As examples,mention may be made of ROV (Roll-Over Valve) valves, i.e. valves thatwill automatically close in the event of the tank turning upside down,ORVR (On-board Refuelling Vapour Recovery) valves, i.e. devices forvapour recovery when refuelling the tank, and OBD (On-Board Diagnostic)devices, and liquid/vapour separators.

Some of these accessories may include at least one pipe passing throughthe sheet via an orifice drilled in the latter. It is often advantageousin this case to provide the outlet orifice of the pipe with a seal.

The sheet may also have been formed so as to be tailored as far aspossible to the surroundings of the opening to be closed off and to theshape and size of the accessory to which it is fastened.

A sheet fastened to a liquid-vapour separator that includes a vapourdischarge pipe passing through the sheet via an orifice has given goodresults.

One particular closure system consists in using a tank having a wallthat bears fixing lugs near the opening. These allow the wall to besupported during the welding operation, should this wall undergoflexible deformation.

The invention also relates to a method of closing off an opening in aplastic tank by means of a plastic sheet of multilayer structure, theouter layer of which, welded to the wall of the tank around theperimeter of the opening, has a composition compatible with that of theplastic forming the outer surface of the tank, in which a peripheralregion of the sheet is slimmed down, before welding, so as to adapt itsgeometry according to the following relationship:l/e≧5in which l represents the width of the slimmed-down region and e itsthickness, both expressed in the same length units.

In this method, the specific terms and expressions that are common, suchas tank, opening, closure, plastic, multilayer structure, sheet,welding, around the perimeter of the opening, compatible composition,have exactly the same meanings as those mentioned above in the case ofthe closure system.

The sheet used in this method may have come from various sources. Forexample, it may have been manufactured independently of the tank itself,possibly at a different time, by means of a specific process that is notnecessarily identical to that used to manufacture the tank. For example,the sheet may be obtained by coextrusion, optionally followed bycompression moulding. It may also have been obtained by moulding usingone of the many known techniques. Such techniques are, for example, blowmoulding, compression moulding, injection moulding and thermoforming.

When the sheet is manufactured independently of the tank, an opening ismade in the tank with dimensions that are slightly smaller than those ofthe sheet, and then the opening is closed off by welding the sheet.

As a variant, the sheet may have been manufactured at the same time asthe tank and cut from the plastic of multilayer structure used tomanufacture the tank. This procedure is particularly advantageous fromthe standpoint of synchronising the production of tanks fitted withtheir accessories. It is possible, for example, to cut out the sheetdirectly from a region of the wall of the tank, after the latter or partof it has been produced. This variant assumes that the innermost outerlayer of the multilayer structure of the tank is compatible with itsoutermost layer.

Another variant is to cut out the sheet from tank manufacturing scrap,provided that this has the appropriate multilayer structure.

It is also possible to cut out the sheet from an ancillary part of thetank specially designed for this purpose and produced simultaneouslywith it.

Finally, it is also possible to manufacture sheets comprising twocomplete structures identical to the structure of the walls of the tank,by pressing together two structures identical to the walls of the tank.In this case, it is possible, for example, to compress a particular partof the parison used for producing the tank. To do this, it is mostparticularly advantageous for the two identical structures of the sheetto be compressed using the same production mould as for the tank.

Any suitable technique may be used to produce the slimmed-downperipheral region of the sheet. Techniques that are well suited areforming techniques such as thermoforming and extrusion-compressionmoulding.

The operation of welding the sheet to the tank may be carried out by anytechnique generally known per se. For example, it is possible to use thehot-plate welding technique or the laser welding technique. Thehot-plate welding technique is preferred. The technique of welding theplate to the tank may be different from that of the slimmed-downperipheral region of this sheet.

According to the invention, the slimmed-down part of the sheet is weldedby a laser radiation or infrared radiation welding technique.

According to a preferred way of implementing the method according to theinvention, a composition opaque to the laser radiation or to theinfrared radiation is incorporated into the plastic layer of theslimmed-down region in contact with the outer surface of the tank.

Advantageously, this method of implementation produces tanks in whichthe welding by laser or infrared radiation of the slimmed-downperipheral region of the sheet is of better quality. The expression“composition opaque to laser or infrared radiation” is understood tomean a composition that absorbs and/or reflects a substantial proportionof the laser or infrared radiation energy. An example of such acomposition that has given good results is a carbon black filler. Thiscomposition is advantageously distributed uniformly throughout thepolymeric material of the layer directly in contact with the wall of thetank.

According to one particular version of the method according to theinvention, at least one accessory is fastened to the sheet, on theinside of the tank. Any fastening method compatible with the tank, thenature of the liquid and the gas that it contains, and with itsoperating conditions may in general be suitable.

However, it is preferred to use a welding process to carry out thisfastening operation.

Here again, the terms and expressions “fastening” and “inside of thetank” have the same meaning as already explained above in the case ofthe closure system.

In this particular way of implementing the method according to theinvention, a first variant consists in welding at least one accessory tothe sheet before the opening in the tank is closed off by welding thesheet fastened to the accessory around the perimeter of this opening.

In this way, the sheet acts as support for the accessory and allows itto be easily introduced into the tank.

A second variant consists, in contrast, in fastening, in a first step,at least one accessory to the internal wall of the tank that is adjacentto the opening and, in a subsequent second step, in closing off the tankby simultaneously welding the sheet to the accessory and to theperimeter of the opening.

In one or other of the methods of implementation employed, one problemmay arise when carrying out the operation of welding the sheet to thewall of the tank when this wall undergoes, for any reason, flexibledeformation near the opening, for example owing to the effect of its ownweight, as a result of forces exerted perpendicular to the surface ofthe tank during welding. A similar situation may in fact arise when thetank comes directly from manufacture and when the temperature of itswalls is still relatively high. Another reason could be that thediameter of the opening is relatively large and that the plastic nearthe edge of the opening is no longer supported as effectively as in thecase of small openings.

The invention also relates to a closure method that remedies theseproblems of flexible deformation of the tank wall, by supporting thetank wall in the region of the opening during the operation of weldingthe sheet by means of lugs moulded to this wall.

The lugs are moulded to the outer wall of the tank near the perimeter ofthe opening. The wall of the tank is supported at this point by means ofclamps that grip the lugs.

Lugs similar to those described above may also be used when the sheet isbeing cut out from the wall of the tank, so as to prevent this wall fromdeforming.

The lugs may also be used, before the welding operation, to correct alack of flatness that may occur on the surface of the tank.

Finally, they may also help to bring the shape of the outer surface ofthe tank into perfect correspondence with that of the sheet.

The invention also relates to the use of the method described above toclose off an opening in a fuel tank, particularly for a motor vehicle.

The term “motor vehicle” is understood to mean vehicles powered by aninternal combustion engine such as lorries, cars and motorcycles.

The FIGURE that follows is given for the purpose of illustrating theinvention, without wishing to restrict its scope.

The FIGURE shows a tank (11), the wall of which is formed from amultilayer structure comprising an EVOH barrier layer surrounded by twoHDPE layers. The barrier layer is fastened to the HDPE layers via a thininterlayer of 80 μm of an adhesive made of a maleic-anhydride-graftedpolyethylene of the ADMER® L2100 trade mark from Mitsui. The multilayerstructure of the wall of the tank (11) has an opening into which asiphon (2) has been snap-fastened onto the rim (5) of the wall (4) ofthe tank (11). A venting ROV valve having a float (6), and provided witha polyacetal skirt (3), was fixed to the siphon (2), again bysnap-fastening.

A sheet (1), of multilayer structure comprising an EVOH barrier layer(9), a HDPE layer (8) and a layer (10) made of a HDPE filled with 0.2%by weight of carbon black, was moulded by thermoforming so as to obtaina slimmed-down profile around a peripheral region (13), (14) with awidth (l) of 10 mm until a thickness (e) of 0.8 mm was obtained in thecase of the layer of carbon-black-filled HDPE. This sheet (1) was weldedto the outer surface (12) of the tank by hot-plate welding, thecarbon-black-filled layer (10) being placed in contact with the wall ofthe tank (11). The slimmed-down peripheral region (13) (14) of the sheet(1) was then welded by a laser beam being scanned over the outer partsof the tank adjoining the hot-plate-welded region (12). The laseremployed was a FAP-type YAG diode laser with a wavelength of 809 nm anda power of 35 W, used in pulsed mode.

1. A method of closing off an opening in a tank, said method comprising:providing a tank defining said opening and having an outer layer ofplastic; providing a plastic sheet of multilayer structure having anouter layer with a composition compatible with that of said plastic ofsaid outer layer of said tank; slimming down a peripheral region of saidplastic sheet according to the following relationship:l/e≧5 in which 1 represents a width of the slimmed-down peripheralregion and e a thickness of a layer of the plastic sheet configured tocontact the wall of the tank, both expressed in a same length unit; andafter said slimming down, welding said plastic sheet to said tank,wherein said welding comprises: hot-plate welding a non-slimmed-downportion of the plastic sheet to said tank, and laser beam welding saidslimmed-down peripheral region to said tank.
 2. The method according toclaim 1, further comprising forming the tank from a multilayerstructure.
 3. The system method according to claim 2, further comprisingforming the tank and the sheet from the same multilayer structure. 4.The method according to claim 1, wherein the l/e ratio is at least
 8. 5.The method according to claim 1, further comprising fastening the sheetto at least one accessory located on the inside of the tank.
 6. Themethod according to claim 1, further comprising incorporating acomposition opaque to laser radiation into the slimmed-down peripheralregion.
 7. The method according to claim 1, wherein the peripheralregion of the sheet is slimmed down using a process chosen fromthermoforming and extrusion-compression moulding.
 8. The methodaccording to claim 1, wherein said tank is a fuel tank for a motorvehicle.
 9. The method according to claim 1, wherein the layer of theplastic sheet in contact with the wall of the tank is flat in theslimmed down region.
 10. The method according to claim 1, wherein thelayer of the plastic sheet in contact with the tank includes a highdensity polyethelyne filled with 0.2% by weight of carbon black.
 11. Themethod according to claim 1, wherein the multilayer structure of saidplastic sheet further comprises a barrier layer that includesethylene/vinyl alcohol random copolymers.
 12. The method according toclaim 1, wherein the multilayer structure of said plastic sheet furthercomprises a barrier layer includes resins based on polyamides orcopolyamides.
 13. The method according to claim 1, wherein themultilayer structure of said plastic sheet further comprises a barrierlayer includes thermotropic liquid-crystal polymers.